It is known from U.S. Pat. No. 5,190,107 to install oilfield equipment at the water bottom by means of a submerged buoy from which the equipment is suspended by a lifting line and which is connected to a work vessel by a J-shaped catenary chain, that compensates for heave motions of the floating work vessel.
The known heave compensated submerged buoy can be used to install relatively small subsea oilfield equipment, such as valves of subsea pipelines and modules of subsea well templates, and the installation may be monitored by a camera on a Remotely Operated Vehicle (ROV) and assisted by a robotic arm mounted on the ROV.
In the known method the equipment is lowered to the water bottom by paying out the J-shaped catenary chain from a winch at the work vessel so that the chain pulls down the submerged buoy.
A disadvantage of the known method is that there is an upper practical limit of the weight of the equipment to be installed at the water bottom. The weight of the equipment determines the size and buoyancy of the buoy and after installation of the equipment the J-shaped catenary chain has to be paid out further until the lifting line is slackened to prevent the buoy from accelerating to surface when the installed equipment is released from the buoy. This implies that if a large piece of equipment is to be installed at the water bottom the submerged buoy has to be connected to the equipment by a long lifting line. The required increased length of the lifting line reduces the depth at which the buoy can be submerged during the installation procedure and results in an imprecise positioning of the equipment at the water bottom.
It is an object of the present invention to alleviate this disadvantage and to provide a method and assembly which is suitable for installing large pieces of underwater oilfield equipment, such as a subsea template of several thousand metric tones, in a more accurate and quick manner than the known method.